This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-070606, filed Mar. 13, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a silver halide emulsion which has high sensitivity and whose fog increase with the passage of storage time has been reduced. The present invention also relates to a silver halide color photographic lightsensitive
2. Description of the Related Art
In recent years, in rivalry with the spread of digital cameras, the requirements for a silver halide emulsion for photography are becoming stricter, and there is a demand for further enhancement of sensitivity and image quality. In particular, even with respect to cheap cameras whose strobe light quantity is likely to be insufficient, such as lens-equipped films being spread, there is a strong demand for a practicable color photographic lightsensitive material of high sensitivity and high image quality. The use of tabular grains is known as providing a technology for attaining a sensitivity increase and an image quality enhancement for the silver halide emulsion. As advantages thereof, a sensitivity increase, including an enhancement of color sensitization efficiency, by the use of a sensitizing dye; an improvement of sensitivity/granularity ratio relationship; a sharpness increase attributed to specific optical characteristics of tabular grains; and an increase of covering power, are known in the art to which the present invention pertains. Generally, when the volumes are identical, an increase in the aspect ratio of tabular grains is advantageous from the viewpoint of an enhancement of sensitivity/granularity ratio.
However, it has been found that reducing the thickness of grains in order to increase the aspect ratio so that a sensitivity increase may be attained would bring about such a problem that the fog occurring with the passage of storage time is intensified. When the fog of a silver halide photographic lightsensitive material in the form of a product is intensified with the passage of time, a balance between sensitivity and gradation would be lost to thereby cause a practical detriment.
Technologies for forming tabular grains of reduced thickness are disclosed in, for example, U.S. Pat. Nos. 5,494,789 and 5,503,970.
With respect to the grain structure relating to silver iodide content phases in the interior of the grains, technologies therefor are disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as JP-A-) 11-153841.
However, in these patent application specifications relating to tabular grains, there is no description regarding the emulsion defined in the claims of the present invention.
It is objects of the present invention to provide an advantageous silver halide photographic emulsion which has high sensitivity and whose fog increase with the passage of storage time has been reduced, and to provide a photographic lightsensitive material using the silver halide photographic emulsion.
The inventors noted the grain iodide structure (in particular, the presence of phases of high silver iodide content) in grain fringe portions in pursuit of an increase of sensitivity of silver halide emulsion by reducing the thickness of tabular grains. Thus, the inventors have found for the first time that a high-sensitivity emulsion whose fog increase with the passage of storage time has been reduced can be obtained by the use of xe2x80x9ca tabular grain whose fringe portion meets the following requirements:
(a) the fringe portion has a phase of high silver iodide content in either one of an upper region and a lower region than a region sandwiched between two twin planes, and
(b) xe2x80x9cAxe2x80x9d, which represents maximum local silver iodide content (mol %) in the phase of high silver iodide content, satisfies the relationship:
Axe2x88x926.0xe2x89xa7B,
wherein xe2x80x9cBxe2x80x9d represents local silver iodide content (mol %) in a part which is positioned on a straight line passing through the part having the maximum local silver iodide content and being perpendicular to the main plane, the part being positioned in the midpoint between the main plane and the twin plane that are opposite, against the region sandwiched between the two twin planes, to the phase of high silver iodide contentxe2x80x9d.
In the aforementioned tabular grain, the fringe portion refers to a grain peripheral portion extending from a grain side defining edge to an inside as much as a length corresponding to a grain thickness, when viewed in a direction perpendicular to the grain main planes. As a result of the finding, the inventors have realized a striking effect which has been unexpectable.
The above grain iodide structure would influence the chemical sensitization, latent image formation and developability of silver halide grains, and would exert effective action in these respects.
The inventors have conducted extensive and intensive investigations. As a result, the inventors have attained effectively both a sensitivity increase and an improvement of fogging with the passage of storage time for a silver halide photographic emulsion and a photographic lightsensitive material, which has been unattainable in the prior art, through the following means.
The means comprise the following silver halide photographic emulsion and lightsensitive material including the same.
(1) A silver halide photographic emulsion comprising grains, wherein 50% or more (numerical ratio) of all the grains are occupied by tabular grains each meeting the requirements (i) to (iii) below:
(i) a silver iodobromide or silver iodochlorobromide tabular grain having (111) faces as main planes and having two parallel twin planes;
(ii) a thickness of 0.12 xcexcm or less; and
(iii) the tabular grains each have a grain fringe portion meeting the following requirements (a) and (b), the grain fringe portion being a grain peripheral portion extending from a grain side defining edge to an inside as much as a length corresponding to a grain thickness, when viewed in a direction perpendicular to the main planes:
(a) the grain fringe portion has a phase of high silver iodide content in either one of an upper region and a lower region than a region sandwiched between the two twin planes, and
(b) xe2x80x9cAxe2x80x9d, which represents the maximum local silver iodide content (mol %) in the phase of high silver iodide content, satisfies the relationship:
Axe2x88x926.0xe2x89xa7B,
xe2x80x83wherein xe2x80x9cBxe2x80x9d represents local silver iodide content (mol %) in a part which is positioned on a straight line passing through the part having the maximum local silver iodide content and being perpendicular to the main plane, the part being positioned in the midpoint between the main plane and the twin plane that are opposite, against the region sandwiched between the two twin planes, to the phase of high silver iodide content.
(2) The silver halide photographic emulsion as defined in item (1) above, wherein xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d satisfy the relationship:
Axe2x88x928.0xe2x89xa7B.
(3) The silver halide photographic emulsion as defined in item (1) above, wherein the grain thickness recited in the requirement (ii) above is 0.10 xcexcm or less.
(4) The silver halide photographic emulsion as defined in item (1) above, wherein the grain thickness recited in the requirement (ii) above is 0.08 xcexcm or less.
(5) The silver halide photographic emulsion as defined in any of items (1) to (4) above, wherein the tabular grains each meet not only the requirements (i) to (iii) but also the requirement (iv) below:
(iv) the tabular grains each have 10 or more dislocation lines per grain in the fringe portion.
(6) The silver halide photographic emulsion as defined in item (5) above, wherein the number of the dislocation lines per grain recited in the requirement (iv) above is 30 or more.
(7) The silver halide photographic emulsion as defined in any of items (1) to (6) above, wherein the tabular grains occupying 50% or more (numerical ratio) of all the grains are produced through a step, in the course of grain formation, wherein one silver halide phase and another silver halide phase are formed in the upper region and in the lower region than the region sandwiched between the two twin planes in the grain fringe portion, respectively, the silver halide phases having respective local silver iodide content maximum values whose difference is 25 mol % or more.
(8) The silver halide photographic emulsion as defined in any of items (1) to (6) above, wherein the tabular grains occupying 50% or more (numerical ratio) of all the grains are produced through a step, in the course of grain formation, wherein either one of at least a part of the upper region and at least a part of the lower region than the region sandwiched between the two twin planes is scooped out once and thereafter recovered, in the grain fringe portion.
(9) The silver halide photographic emulsion as defined in any of items (1) to (6) above, wherein the tabular grains occupying 50% or more (numerical ratio) of all the grains are produced through a step wherein silver halide epitaxy containing silver iodide is formed at a position on either the upper region or the lower region of the grain fringe portion, in the course of grain formation.
(10) The silver halide photographic emulsion as defined in any of items (1) to (6) above, wherein the tabular grains occupying 50% or more (numerical ratio) of all the grains are produced through a step wherein iodide ions are released from an iodide ion-releasing agent, thereby forming epitaxy containing silver iodide in the course of grain formation.
(11) A silver halide color photographic lightsensitive material comprising at least one layer containing a silver halide emulsion on a support, wherein at least one layer among the at least one layer contains the silver halide photographic emulsion as defined in any of items (1) to (10) above.